mineral
Banded agate (likely quartz-rich chalcedony)
Microcrystalline quartz, SiO₂; possibly agate with a weathered iron-rich host rock
AI-generated identification · may be incorrectHardness 6.5–7 Mohs; gray, white, and brown-black bands with waxy to dull luster. Cryptocrystalline quartz lacks visible crystal faces and has conchoidal fracture; the pale edge appears translucent and finely layered.
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Physical properties
Hardness 6.5–7 Mohs; gray, white, and brown-black bands with waxy to dull luster. Cryptocrystalline quartz lacks visible crystal faces and has conchoidal fracture; the pale edge appears translucent and finely layered.
Formation & geological history
Forms when silica-rich groundwater fills cavities in volcanic rocks, depositing successive chalcedony layers. Individual bands may form over thousands to millions of years; the dark outer material is likely weathered host rock or iron/manganese staining.
Uses & applications
Used for cabochons, carvings, beads, bookends, and collector specimens; attractive polished pieces have modest gemstone value.
Geological facts
Agate is a variety of chalcedony distinguished by banding, often enhanced by iron or manganese oxides. A strong glassy luster and conchoidal fracture help distinguish it from ordinary gray rock.
Field identification & locations
Common in volcanic terrains worldwide, especially Brazil, Uruguay, Mexico, India, Madagascar, and the western United States. Test with a steel file or quartz point, inspect for curved fracture, and avoid acid-based identification tests.
